This invention relates to a magneto-equipped power device, and more particularly to a power device adapted to feed a DC power to a load by means of an output of a magneto.
A power device for a vehicle or the like which is driven by an internal combustion engine is typically constructed so as to convert an output of a magneto driven by the internal combustion engine into a DC power through a rectification circuit.
As commonly known in the art, a magneto is generally constituted of a rotor having a magnet field and a stator including armature coils, wherein the rotor is mounted on a prime mover of an internal combustion engine or the like. In the magneto thus constructed, a variation in magnetic flux produced due to rotation of the rotor permits an AC voltage to be induced across the armature coils.
In a power device using a generator as a power supply, it is generally required to restrain an output of the generator or isolate the generator from a load in order to protect the load. In particular, when a magneto driven by an internal combustion engine mounted on a vehicle or the like is used as a power supply for the power device, the magneto is substantially varied in rotational speed with a variation in rotational speed of the engine. Also, the magneto is increased in output with an increase in rotational speed thereof. Thus, in order to protect the load, it is required to keep an output of the magneto at a predetermined level or below.
Also, even when the magneto is operated while keeping a rotational speed thereof at a constant level, the load is often caused to be excessively increased in voltage across the load or load current or increased in load temperature beyond an allowable level depending on a state of the load, so that it is required to take a step necessary to protect the load.
Conventionally, the power device using the magneto employs two types of output control systems for the purpose of protecting the load. One of the systems is the output short-circuit type wherein an output of the armature coils is short-circuited to restrain an output of the magneto when the load falls into a state of requiring to restrain an output of the magneto. The other system is the load release type wherein the load is released from the magneto when any situation of requiring to protect the load occurs.
The conventional power device employing the output control system of the output short-circuit type includes a rectification circuit for rectifying an output of the magneto to feed it to the load, an output short-circuiting circuit for substantially short-circuiting an output of the armature coils of the magneto through output short-circuiting switches when it is fed with a control signal to flow a DC short-circuit current therethrough, and a switch control circuit for feeding a control signal to the output short-circuit switches when it is detected that the load falls into a state of requiring to restrain an output of the magneto, whereby the output short-circuit switches are turned on when it is detected that the load falls into a state of requiring to restrain an output of the magneto, to thereby short-circuit an output of the magneto, resulting in restraining an output of the magneto.
The power device employing the output control system of the load release type uses a rectification circuit for rectifying an output of the magneto which is constituted of a control rectification circuit using thyristors as switching elements, whereby the thyristors of the control rectification circuit are turned off when a voltage across the load exceeds a predetermined level or a current of the load exceeds a predetermined level, to thereby release the load from the magneto, resulting in the load voltage or load current being kept at the predetermined level or below.
In the conventional power device employing the output control system of the output short-circuit type, the output short-circuit switches are turned on while the magneto generates a voltage of an increased level, leading to flowing of a short-circuit current therethrough, so that it is required to increase a current capacity of the output short-circuit switches, resulting in the power device being increased in cost. Also, this causes an increase in short-circuit current flowing through the output short-circuit switches, leading to an increase in generation of heat from the output short-circuit switches, resulting in requiring arrangement of a large-sized heat sink for the output circuit switches, so that the power device is increased in size.
In the power device employing the output control system of the load release type, the thyristors of the control rectification circuit are kept turned off during a high-speed operation of the internal combustion engine for which the magneto is increased in no-load output voltage, resulting in an increased no-load voltage of the magneto being applied to the thyristors. This needs to increase dielectric properties of the thyristors for the control rectification circuit, leading to an increase in cost of the thyristors and therefore the power device.
Another power device is proposed as disclosed in Japanese Patent Application Laid-Open Publication No. 59319/1995. The power device proposed is so constructed that a field adjusting excitation coil for flowing a magnetic flux through magnetic poles on a side of a rotor (hereinafter referred to as "rotor-side magnetic poles") of a magneto and those on a side of a stator (hereinafter referred to as "stator-side magnetic poles") thereof is arranged on a side of the stator and a magnet field of the rotor is increased when a magnetic flux is fed from the field adjusting excitation coil thereto, whereby the field adjusting excitation coil is excited when a load which is increased to a high level is driven, to thereby increase an output of the magneto. In the power device thus constructed, an output of the magneto is fed through a rectifier to a battery, so that an output voltage of the battery is applied to the load, as well as to the field adjusting excitation coil through a switch. When the load is reduced, the field adjusting excitation coil is released or separated from the battery, to thereby be rendered non-excited, to thereby decrease a magnet field of the rotor, resulting in preventing an output of the magneto from being excessive. When the load is increased, the switch is closed to excite the field adjusting excitation coil by means of an output of the battery, to thereby increase the magnet field, leading to an increase in output of the magneto.
Thus, the power device is constructed so as to excite the field adjusting excitation coil to increase an output of the magneto when the load is increased and render the field adjusting excitation coil non-excited to restrain an output of the magneto when the load is decreased. Such construction permits an output of the generator to be controlled depending on a state of the load without short-circuiting an output of the magneto or isolating the load from the magneto.
However, the power device fails to finely adjust an output of the magneto depending on a magnitude of the load, because an excitation current fed to the field adjusting excitation coil is determined depending on a voltage across the battery charged by the magneto.
In order to finely adjust an output of the magneto, it would be considered to arrange a control circuit including a detection circuit for detecting a load current or a load voltage and an excitation current adjusting circuit for adjusting a magnitude of an excitation current flowing through the field adjusting excitation coil depending on a detection signal outputted from the detection circuit, to thereby control a magnitude of the excitation current. However, arrangement of such a control circuit for controlling a magnitude of an excitation current flowing through the field adjusting excitation coil renders the power device complicated in structure.